This invention relates generally to the manufacture of color cathode ray tubes (CRTs) and is particularly directed to the recovery of flat glass faceplates in CRTs rejected during the manufacturing process for re-use.
A conventional CRT consists of an evacuated envelope having a neck portion, a glass faceplate, and a funnel portion therebetween. An electron gun disposed in the neck portion of the envelope emits energetic electrons which are directed onto the inner surface of the faceplate. Disposed on the inner surface of the faceplate are a large number of phosphor elements which glow momentarily when struck from the rear by electrons from the electron gun to produce a video image which is visible through the faceplate. Also disposed on the inner surface of the faceplate is a layer of a black, light-absorbing material in the form of a matrix which defines the phosphor dots. A metalized layer typically including aluminum is also frequently disposed on the CRT's inner surface to reflect the light produced by the electron-excited phosphor elements outward toward the viewer for enhanced video image brightness.
A CRT is particularly susceptible to the effects of ambient light incident upon its faceplate. Ambient light produces reflections from both the outer, or forward, surface of the CRT's faceplate as well as from its inner, or aft, surface. In the past, the surfaces of the CRT's faceplate have typically been roughened to a given surface texture either by pressing the glass surface, called stippling; by using a chemical process such as acid etching; or have been covered with a layer of anti-reflection coating to reduce reflections directed back toward the viewer which degrade the video image. The pressing and acid etching approaches roughen the surfaces of the faceplate so as to reduce specular reflection from the faceplate and increase its diffuse reflection. The layers of anti-reflection coating deposited on the faceplate's surfaces possess interference properties selected so as to minimize ambient light reflection from its forward and aft surfaces toward the viewer.
Recent developments in CRT technology have lead to the use of flat glass faceplates in color CRTs. These flat glass faceplates have attached to the inner surface thereof a flat tension mask (FTM) which is affixed to a support structure and is maintained in a tightly stretched condition. The FTM is in the form of a thin metal foil having a large number of very small apertures through which the electron beams are directed to permit the FTM to serve as a color selection electrode, or parallax barrier. The FTM support structure may be comprised of metal or a ceramic material and is affixed to the inner surface of the faceplate and attached to the FTM by means of a frit-based cement or by welding, depending upon the support structure composition.
The etched inner surface of the flat glass faceplate has a carefully controlled degree of roughness in order to minimize internal reflections without degrading video image acuity. The degree of roughness is generally expressed as a depression value range, with a roughness range of 10-20 microns root mean square (RMS) depression value range providing optimum video image acuity and minimum internal specular reflection.
During manufacture, the CRT undergoes numerous tests at various stages of assembly. This rigorous testing is intended to ensure that the product sold is of high quality and reliability. However, this demanding test schedule inevitably results in the rejection of CRTs at various stages of assembly. Because of the high cost of some CRT components, it is cost effective to salvage and re-use acceptable components of otherwise rejected CRT. One CRT component which is particularly desirable to salvage and re-use because of its relative cost and because it does not generally contribute to the rejection of the CRT is the flat glass faceplate. Before the flat glass faceplate can be re-installed in another CRT during assembly, it must be restored to its original condition, followed by the application of the various layers of video image producing and enhancing materials on its inner surface. This means that the glass faceplate must be separated from the CRT funnel; the FTM and its support structure must be removed from the glass faceplate; and the phosphor coating, black surround, and aluminized layer must be removed from the faceplate's inner surface.
Prior art approaches for cleaning the flat glass faceplate of a rejected CRT for re-use involve the application of pressurized water with a micro-soap and hot caustic solution to the faceplate. This approach has been of limited use because of the incomplete removal of the aluminized, phosphor and black matrix layers. Incomplete removal of any portion of any of the aforementioned layers from the inner surface of the faceplate renders the faceplate unacceptable for re-installation in a CRT. Acid etching employing hydrofluoric etching acid such as used in initial roughening of the faceplate's surface has proven to be effective in removing the aforementioned materials, but leads to additional etching of the glass surface. This additional etching increases the roughness, or texture, of the glass faceplate rendering it unacceptable for video image transmission.
The present invention addresses and overcomes the aforementioned limitations of the prior art by providing a method for cleaning the roughened inner surface of a flat glass faceplate used in a color CRT. By directing a vapor blast containing small particles or pellets onto the faceplate's inner surface, the various video image producing and enhancing materials disposed on the faceplate are removed. A preferred embodiment contemplates vapor blasting at a pressure on the order of 20-30 p.s.i. with a slurry of ground up walnut shells in water.